Modern power circuits require power rectifiers with improved power switching performance. For some applications, p+/n rectifiers with high switching speeds are used. High switching speeds are necessary to minimize reverse current flow during recovery. Majority carrier devices, such as Schottky barrier rectifiers, are often used because they offer both improved switching speed and lower forward voltage drop. Unfortunately, Schottky barrier rectifiers suffer from undesirably high reverse leakage current when operating at elevated temperatures.
Several improvements have been introduced to improve the blocking ability of Schottky rectifiers. One improvement is the junction barrier Schottky (JBS) rectifier, which combines a p/n junction grid with Schottky barrier regions which are sufficiently small so that the expanding space charge region from the p+/n junction grid leads to the elimination of the Schottky barrier lowering which is otherwise caused by the resulting image charge. For the JBS rectifier, there is a net reduction in leakage current of approximately 50% for the same chip area and forward voltage drop. This equates to an approximately 11 degree improvement in temperature in the power dissipation curve when operating at a 50% duty cycle.
Another improvement is trench Schottky, which is useful for higher voltage applications in which the forward voltage drop exceeds 0.7 volts and the JBS rectifier ceases to operate as a majority carrier device. For example, the trench MOS barrier-controlled Schottky (MBS) rectifier has a lower forward voltage drop than the p-i-n rectifier for breakdown voltages up to 250V, and still operates as a majority carrier device.
In addition to these high voltage applications, however, there is an increasing demand for low voltage applications, for which conventional trench Schottky is not well-suited. Trench Schottky requires that, in the blocking state, the inner trenches are sufficiently closely spaced and the adjacent areas of the body portion are sufficiently lowly doped that the depletion layer formed in the body portion depletes the intermediate areas of the body portion between the trenches at a smaller voltage than the breakdown voltage. In that way, the reverse voltage blocking characteristic is improved. Unfortunately, it also results in a significant reduction of the area available for the Schottky barrier because the trench may consume as much as 50% of the available area on a chip.